Numerous methods are known for detecting and estimating thiols, i.e. compounds containing a mercapto or sulfhydryl group --SH. A particularly valuable and frequently cited method is that described by G. H. Ellman, Arch. Biochem Biophys. 1959, 82, 70-77; also U.S. Pat. No. 3,119,668, which depends upon the formation of yellow colored 3-mercapto-6-nitrobenzoic acid anion by a displacement reaction between a thiol and 3,3'-dithiobis [6-nitrobenzoic acid] (Ellman's reagent). The detection test is extremely sensitive and the reaction with many thiols is very rapid. The method is particularly suited for many instrumental thiol analyses because of the great difference between the molar absorptivities of the reagent and its anion. However, the method is less useful for detection tests which depend on visual observation of a color change, since both Ellman's reagent and the displaced anion (the colored reaction product) are yellow to orange in color. Consequently, it is difficult to interpret data based on a color change in the same hue, particularly when the color change is very small, which is the case when the thiol is present in very low concentrations.
An objective of the present invention is to provide a novel class of compounds, which are suitable for the detection of thiols and in many cases avoid the aforesaid difficulties.